Flue gas heat recovery system

ABSTRACT

A flue gas heat recovery system includes a furnace, a combustion device, a flue gas duct, and a heat exchanging unit, and further includes a heat recovering unit disposed in the flue gas duct on the downstream side of the heat exchanging unit to recover heat from flue gas, and configured such that a path cross-sectional area of an outlet of the heat recovering unit is made smaller than a path cross-sectional area of an inlet of the heat recovering unit, and a first heat exchanger that heats combustion air by using heat recovered by the heat recovering unit.

FIELD

The present invention relates to a flue gas heat recovery system which recovers heat of flue gas discharged from a boiler generating steam by combustion of fuel and air.

BACKGROUND

For example, a coal combustion boiler includes a hollow furnace located in the vertical direction. A plurality of combustion burners are provided in a lower part of the furnace. A flue gas duct is connected with an upper part of the furnace. A heat exchanger is provided in the flue gas duct to recover heat from flue gas. Water is heated by using flue gas generated by combustion within the furnace to generate steam. A gas duct is further connected with the flue gas duct of the coal combustion boiler. An air heater is provided in the gas duct. The air heater heats air by using flue gas to generate heated air, and supplies the heated air to the combustion burners as combustion air.

The air heater rotates a heat element to alternately bring a flue gas path and an air path into contact with flue gas and air, respectively, for heat exchange, thereby heating air by using flue gas for generation of heated air. The flue gas discharged from the boiler contains corrosion substances such as sulfurous acid (SO₃). Accordingly, the amount of heat recovered by the air heater is limited within such a range that sulfurous acid does not become sulfuric acid by condensation. Even when heat is not recovered by the air heater to such a temperature not causing condensation of sulfurous acid, an area having a temperature lowered to a condensation temperature of sulfurous acid may be produced due to the mechanism of heat exchange realized by rotation of the heat element. This area may cause corrosion or closure of the heat element of the air heater.

For example, Patent Literature 1 describes a coal combustion boiler which includes a heat recovery device in place of the rotational air heater. This heat recovery device includes a high-temperature loop and a low-temperature loop. In the high-temperature loop, a high-temperature heat medium circulates to preheat combustion air by using heat recovered from flue gas, while in the low-temperature loop, a low-temperature heat medium circulates to preheat combustion air by using heat recovered from flue gas, and further to reheat flue gas and heat boiler feed water.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Laid-open Patent Publication No. 63-217103

SUMMARY Technical Problem

In the heat recovery device described above, the heat medium is heated by using heat of flue gas. The heated heat medium heats combustion air, flue gas, and boiler feed water. In this case, the temperature of the flue gas at an inlet of the heat recovery device is 300° C. or higher, while the temperature of the flue gas at an outlet decreases to 100° C. or lower. In this case, a temperature change within the heat recovery device becomes 200° C. or higher. Accordingly, the volume of the flue gas flowing through the heat recovery device considerably changes (decreases), and the flow velocity also considerably changes (decreases). In this condition, the flow velocity of the flue gas increases at the inlet of the heat recovery device. As a result, erosion of a heat exchange tube may be accelerated. On the other hand, the flow velocity of the flue gas decreases at the outlet of the heat recovery device. In this case, heat exchange performance may be lowered, while soot and dust or the like may be accumulated.

The present invention has been developed to solve the aforementioned problems. It is an object of the present invention to provide a flue gas heat recovery system capable of increasing durability and heat recovery efficiency.

Solution to Problem

According to an aspect of the present invention, a flue gas heat recovery system comprises: a hollow furnace located in a vertical direction; a combustion burner that blows fuel gas constituted by a mixture of fuel and combustion air toward the furnace; a flue gas path connected to an upper part of the furnace; a heat exchanging unit disposed in the flue gas path to exchange heat between flue gas and water; a heat recovering unit disposed in the flue gas path on the downstream side of the heat exchanging unit and configured such that a path cross-sectional area of an outlet of the heat recovering unit is made smaller than a path cross-sectional area of an inlet of the heat recovering unit; and a first heat exchanger that heats combustion air by using heat recovered by the heat recovering unit.

According to this structure, a heat medium recovers heat from flue gas while the flue gas is passing through the heat recovering unit. In this case, the temperature and volume of the flue gas decrease. However, the heat recovering unit is configured such that the path cross-sectional area of the outlet of the heat recovering unit is made smaller than the path cross-sectional area of the inlet. Accordingly, the flow velocity does not increase when the flue gas having the large volume passes through the inlet. On the other hand, the flow velocity does not decrease when the flue gas having the small volume passes through the outlet. In other words, fluctuations of the flow velocity are reduced even when the temperature and volume of the flue gas decrease during passage through the heat recovering unit. As a result, erosion of the heat exchange tube constituting the heat recovering unit, and deterioration of the heat exchange performance are both avoidable. Accordingly, durability and heat recovery efficiency are improved.

Advantageously, in the flue gas heat recovery system, the heat recovering unit is configured such that an inner size of the outlet in the flue gas path is made smaller than an inner size of the inlet.

According to this structure, the path cross-sectional area of the inlet of heat exchanger into which high-temperature flue gas flows is determined easily and in a simplified configuration, so that the flue gas flow velocity becomes a velocity appropriate for reducing erosion of the heat exchange tubes, and for efficiently recovering heat also from low-temperature flue gas after heat exchange on the upstream side.

Advantageously, in the flue gas heat recovery system, the heat recovering unit is configured such that density of a heat exchange tube or heat exchange fins at the outlet is made higher than density of the heat exchange tube or the heat exchange fins at the inlet.

According to this structure, the path cross-sectional area of the inlet of heat exchanger into which high-temperature flue gas flows is easily determined so that the flue gas flow velocity becomes a flow velocity appropriate for reducing erosion of the heat exchange tubes, and for efficiently recovering heat also from low-temperature flue gas after heat exchange on the upstream side, in accordance with a change of the position, shape, and number of the heat exchange tube or the heat exchange fins, without the necessity of changing the configuration of the flue gas path.

Advantageously, in the flue gas heat recovery system, the heat recovering unit includes an upstream side high temperature unit and a downstream side low temperature unit, and is configured such that a path cross-sectional area of the low temperature unit is made smaller than a path cross-sectional area of the high temperature unit.

According to the structure of the heat recovering unit including the high temperature unit and the low temperature unit, the path cross-sectional area of the inlet of heat exchanger into which high-temperature flue gas flows is easily determined so that the flue gas flow velocity becomes a flow velocity appropriate for reducing erosion of the heat exchange tubes, and for efficiently recovering heat also from low-temperature flue gas after heat exchange on the upstream side.

Advantageously, in the flue gas heat recovery system, the flue gas path includes a vertical path extending in the vertical direction and a horizontal path connected with a lower part of the vertical path and extending in a horizontal direction, the high temperature unit is disposed in the vertical path, and the low temperature unit is disposed in the horizontal path.

According to the structure which positions the high temperature unit in the vertical path and the low temperature unit in the horizontal path, the path cross-sectional area of the inlet of heat exchanger into which high-temperature flue gas flows is determined easily and in a simplified configuration, so that the flue gas flow velocity becomes a flow velocity appropriate for reducing erosion of the heat exchange tubes, and for efficiently recovering heat also from low-temperature flue gas after heat exchange on the upstream side, without the necessity for changing the configuration of the existing flue gas path.

Advantageously, in the flue gas heat recovery system, a NOx removal device is provided in the vertical path, and the high temperature unit is disposed below the NOx removal device.

According to this structure, the high temperature unit is disposed below the NOx removal device. In this case, flue gas flows into the high temperature unit after removal of harmful substances by the NOx removal device. Accordingly, adhesion of harmful substances to the heat recovering unit is avoidable.

Advantageously, in the flue gas heat recovery system, a hopper is provided between the vertical path and the horizontal path and below the high temperature unit.

According to this structure, the hopper is disposed below the high temperature unit. Accordingly, particles such as soot and dust contained in flue gas are collectable between the vertical path and the horizontal path.

Advantageously, the flue gas heat recovery system further comprises a second heat exchanger that reheats flue gas prior to discharging from a stack by using heat recovered by the heat recovering unit.

According to this structure, the second heat exchanger reheats flue gas by using heat recovered by the heat recovering unit. Accordingly, generation of white smoke is avoidable.

Advantageously, the flue gas heat recovery system further comprises a third heat exchanger that heats water supplied to the heat exchanging unit by using heat recovered by the heat recovering unit.

According to this structure, the third heat exchanger heats water supplied to the heat exchanging unit by using heat recovered by the heat recovering unit. Accordingly, effective utilization of the recovered heat is achievable.

Advantageously, the flue gas heat recovery system further comprises a distribution amount control device that adjusts distribution amounts of a heat medium supplied from the heat recovering unit to the first heat exchanger, the second heat exchanger, and the third heat exchanger.

According to this structure, the distribution amount control device adjusts the distribution amounts of the heat medium supplied from the heat recovering unit to the respective heat exchangers in accordance with the operation condition of the boiler. Accordingly, this structure makes it possible, while keeping a necessary amount of heat recovered from flue gas for environmental measurements, to utilize the remaining amount of heat recovered for improvement of power generation efficiency and for appropriate operation of the unit.

A flue gas heat recovery system according to the present invention includes a heat recovering unit disposed in a flue gas path for recovery of heat from flue gas, and configured such that a path cross-sectional area at an outlet of the heat recovering unit is made smaller than a path cross-sectional area at an inlet of the heat recovering unit. Accordingly, durability and heat recovery efficiency is increased.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view schematically illustrating a configuration of a boiler which includes a flue gas heat recovery system according to a first embodiment.

FIG. 2 is a view schematically illustrating flows of water (steam) and a heat medium included in the flue gas heat recovery system.

FIG. 3 is a view schematically illustrating a heat recovering unit and heat exchangers.

FIG. 4 is a view schematically illustrating a configuration of the heat recovering unit.

FIG. 5 is a view schematically illustrating a heat recovering unit and heat exchangers included in a flue gas heat recovery system according to a second embodiment.

FIG. 6 is a view schematically illustrating a heat recovering unit and heat exchangers included in a flue gas heat recovery system according to a third embodiment.

FIG. 7 is a view schematically illustrating a heat recovering unit and heat exchangers included in a flue gas heat recovery system according to a fourth embodiment.

FIG. 8 is a view schematically illustrating a heat recovering unit included in a flue gas heat recovery system according to a fifth embodiment.

FIG. 9 is a view schematically illustrating a fin tube of the heat recovering unit.

FIG. 10 is a view schematically illustrating a heat recovering unit and heat exchangers included in a flue gas heat recovery system according to a sixth embodiment.

DESCRIPTION OF EMBODIMENTS

A flue gas heat recovery system according to preferred embodiments according to the present invention is hereinafter described in detail with reference to the accompanying drawings. The present invention is not limited to the embodiments described herein. When there are presented a plurality of embodiments, arbitrary combinations of the respective embodiments are contained in the scope of the present invention.

First Embodiment

FIG. 1 is a view schematically illustrating a configuration of a boiler which includes a flue gas heat recovery system according to a first embodiment.

The boiler in which a flue gas heat recovery system according to the first embodiment is included is a coal combustion boiler which uses pulverized coal as pulverized fuel (fuel), and burns the pulverized fuel by using combustion burners. The flue gas heat recovery system recovers heat generated by the combustion of the pulverized fuel.

As illustrated in FIG. 1, a boiler 10 in the first embodiment is a conventional boiler which includes a furnace 11, a combustion device 12, a flue gas duct (flue gas path) 13, and a heat exchanging unit 14. First of all, a general configuration of the boiler 10 is described.

The furnace 11 has a hollow and square cylindrical shape, and is located in the vertical direction. A furnace wall constituting the furnace 11 includes a heat exchange tube.

The combustion device 12 is disposed in a lower part of the furnace wall constituting the furnace 11. The combustion device 12 includes a plurality of stages of combustion burners 21, 22, 23, 24, and 25 attached on the furnace wall. According to this embodiment, five sets, or five stages of the combustion burners 21, 22, 23, 24, and 25, each of which contains four burners at equal intervals in the circumferential direction as one set, are provided in the vertical direction. However, the shape of the furnace 11, the number of the combustion burners for each stage, the number of stages of the combustion burners, and other conditions are not limited to the specific examples in this embodiment, but may be arbitrarily determined.

Each of the combustion burners 21, 22, 23, 24, and 25 uses pulverized coal which is milled coal as solid fuel. Coal pulverizers (pulverizers or mills) 31, 32, 33, 34, and 35 are connected with the combustion burners 21, 22, 23, 24, and 25 via pulverized coal supply tubes 26, 27, 28, 29, and 30, respectively. While not depicted in the figure, the coal pulverizers 31, 32, 33, 34, and 35 pulverize coal into a predetermined size when coal is supplied between a plurality of pulverizing rollers and a pulverizing table. Pulverized coal thus produced is classified by using conveyance air (primary air), and supplied to the combustion burners 21, 22, 23, 24, and 25 via the pulverized coal supply tubes 26, 27, 28, 29, and 30.

The furnace 11 includes a wind box 36 disposed at attachment positions of the combustion burners 21, 22, 23, 24, and 25. The furnace 11 further includes an additional air nozzle 37 on the furnace wall at a position above the attachment positions of the combustion burners 21, 22, 23, 24, and 25. One end of an air duct 38 is connected with a blower 39, while the other end is connected with the wind box 36 and the additional air nozzle 37. Accordingly, combustion air (secondary air) generated by the blower 39 is supplied to the wind box 36 via the air duct 38, and further supplied from the wind box 36 to the respective combustion burners 21, 22, 23, 24, and 25. The combustion air coming from the air duct 38 is further supplied to the additional air nozzle 37.

The flue gas duct 13 is connected with an upper part of the furnace 11. The flue gas duct 13 includes a first horizontal path 41 connected with an upper end of the furnace 11, a first vertical path 42 connected with an end of the first horizontal path 41, a second horizontal path 43 connected with a lower end of the first vertical path 42, a second vertical path 44 connected with an end of the second horizontal path 43, and a gas duct 45 connected with an end of the second vertical path 44.

The flue gas duct 13 includes the heat exchanging unit 14 in the first horizontal path 41 and the first vertical path 42. The heat exchanging unit 14 exchanges heat between flue gas generated by combustion within the furnace 11 and water (steam) flowing within the heat exchange tube. The heat exchanging unit 14 includes superheaters 46, 47 and 48, reheaters 49 and 50, and economizers (economizers) 51 and 52.

The flue gas duct 13 includes a NOx removal device (selectively reducing catalyst) 61 disposed in the second vertical path 44. The NOx removal device 61 decomposes nitrogen oxide (NOx) contained in flue gas into harmless nitrogen and steam by the function of a catalyst contained in a reducing agent (such as ammonia)

The flue gas duct 13 includes an electrostatic precipitator 62, an induced draft fan 63, a desulfurization device 64, and a stack 65 in the gas duct 45. The electrostatic precipitator 62 charges particles of various types of dust contained in flue gas to attract the particles toward a collecting electrode and collect dust. The desulfurization device 64 is a wet-type desulfurization device that ejects absorbent toward flue gas having entered an absorber and brings the absorbent into contact with the flue gas to absorb and remove sulfur oxide (SO2) gas contained in the flue gas.

The boiler 10 according to this embodiment includes a heat recovering unit 71 disposed in the flue gas duct 13 on the downstream side of the heat exchanging unit 14 to recover heat from flue gas. The heat recovering unit 71 includes an upstream side high temperature unit 72 and a downstream side low temperature unit 73. The high temperature unit 72 and the low temperature unit 73 are directly connected via a heat exchange tube. The boiler 10 further includes a first heat exchanger 74 which heats combustion air by using heat recovered by the heat recovering unit 71. More specifically, provided between the heat recovering unit 71 and the first heat exchanger 74 is a heat medium circulation path 75 where a heat medium (such as water and steam) circulates. The heat medium circulation path 75 includes a first circulation path 75 a extending from the high temperature unit 72 to the first heat exchanger 74, and a second circulation path 75 b extending from the first heat exchanger 74 to the low temperature unit 73. A pump 76 is provided in the second circulation path 75 b to circulate the heat medium.

The boiler 10 further includes a second heat exchanger 77 which reheats flue gas prior to discharge from the stack 65 by using heat recovered by the heat recovering unit 71. The boiler 10 further includes a third heat exchanger 78 (see FIG. 2) which heats feed water supplied to the economizers 51 and 52 of the heat exchanging unit 14 by using heat recovered by the heat recovering unit 71.

According to this structure, pulverized coal is produced by operation of the coal pulverizers 31, 32, 33, 34, and 35, and supplied from the pulverized coal supply tubes 26, 27, 28, 29, and 30 to the combustion burners 21, 22, 23, 24, and 25 by using conveyance air. The combustion air heated by the first heat exchanger 74 is supplied from the air duct 38 through the wind box 36 to the respective combustion burners 21, 22, 23, 24, and 25, and also supplied to the additional air nozzle 37. The pulverized coal mixture air containing a mixture of the pulverized coal and the conveyance air, and the combustion air are both blown into the furnace 11 and ignited in this condition by the combustion burners 21, 22, 23, 24, and 25 to produce flames. In addition, additional air is blown through the additional air nozzle 37 into the furnace 11 for combustion control so as to reduce NOx generated by combustion of the pulverized coal. Thereafter, flue gas having passed through the heat exchanging unit 14 of the flue gas duct 13 is discharged through the stack 65 into the atmosphere after removal of NOx by the NOx removal device 61, particulate substances by the electronic precipitator 62, and sulfur oxide by the desulfurization device 64.

Flows of water (steam) and the heat medium in the flue gas heat recovery system are hereinafter described. FIG. 2 is a view schematically illustrating flows of water and the heat medium in the flue gas heat recovery system, while FIG. 3 is a view schematically illustrating a heat recovery device and heat exchangers.

As illustrated in FIGS. 2 and 3, the heat recovering unit 71 includes the high temperature unit 72 and the low temperature unit 73. The high temperature unit 72 and the low temperature unit 73 contain heat exchange tubes 72 a and 73 a, respectively. One end of the heat exchange tube 72 a is connected with one end of the heat exchange tube 73 a. The first heat exchanger 74 contains a heat exchange tube 74 a. The other end of the heat exchange tube 72 a and one end of the heat exchange tube 74 a are connected via a first circulation path 75 a. The other end of the heat exchange tube 73 a and the other end of the heat exchange tube 74 a are connected via a second circulation path 75 b. The pump 76 is provided in the second circulation path 75 b.

The second heat exchanger 77 contains a heat exchange tube 77 a. A first branch path 81 a branched from the first circulation path 75 a is connected with one end of the heat exchange tube 77 a. A second branch path 81 b branched from the second circulation path 75 b is connected with the other end of the heat exchange tube 77 a. The third heat exchanger 78 contains a heat exchange tube 78 a. A third branch path 82 a branched from the first branch path 81 a is connected with one end of the heat exchange tube 78 a. A fourth branch path 82 b branched from the second branch path 81 b is connected with the other end of the heat exchange tube 78 a.

A steam turbine 91 operated by steam generated from the boiler includes a high-pressure turbine 92 and a low-pressure turbine 93. A water/steam circulation path 94 is provided between the heat exchanging unit 14 of the boiler 10 and the steam turbine 91 to circulate water and steam. The superheaters 46, 47, and 48, the high-pressure turbine 92, the reheaters 49 and 50, the low-pressure turbine 93, a condenser 95, the third heat exchanger 78, a deaerator 96, a water feed pump 97, and the economizers 51 and 52 are provided in this order in the water/steam circulation path 94.

Accordingly, flue gas (300° C. to 400° C.) flowing in the flue gas duct 13 proceeds in the order of the high temperature unit 72 and the low temperature unit 73 of the heat recovering unit 71. During the flow of the flue gas in the flue gas duct 13, the heat recovering unit 71 recovers heat from the flue gas by using the heat medium. More specifically, the heat medium (65° C. to 100° C.) is circulated in the heat medium circulation path 75 in accordance with operation of the pump 76, and heated by the heat of the flue gas during circulation. Thereafter, a part of the high-temperature heat medium (100° C. to 350° C.) is supplied to the first heat exchanger 74 to exchange heat between the heat medium circulating in the heat medium circulation path 75 and the air flowing in the air duct 38. As a result, the air heated by the heat medium becomes high-temperature air, and is supplied to the combustion device 12 as combustion air (200° C. to 330° C.)

On the other hand, a part of the high-temperature heat medium generated by the heat recovering unit 71 is supplied to the second heat exchanger 77 to exchange heat between the high-temperature heat medium and the flue gas (40° C. to 70° C.) discharged from the desulfurization device 64. The flue gas (80° C. to 100° C.) reheated by the heat medium is supplied to the stack 65. Furthermore, a part of the high-temperature heat medium generated by the heat recovering unit 71 is supplied to the third heat exchanger 78 to exchange heat between the high-temperature heat medium and the water (30° C. to 70° C.) flowing in the water/steam circulation path 94. The water heated by the heat medium becomes high-temperature water (60° C. to 100° C.), and is supplied to the economizers 51 and 52.

On the other hand, water supplied from the water feed pump 97 is preheated by the economizers 51 and 52, and supplied to a not-shown steam drum. This water is heated during supply to the respective heat exchange tubes of the furnace wall to become saturated steam, and supplied to the steam drum in a state of saturated steam. The saturated steam of the steam drum is introduced into the superheaters 46, 47, and 48, and superheated by flue gas. The superheated steam generated by the superheaters 46, 47, and 48 is supplied to the high-pressure turbine 92 to drive the high-pressure turbine 92. Steam discharged from the high-pressure turbine 92 is introduced into the reheaters 49 and 50 and again superheated, and then supplied to the low-pressure turbine 93 to drive the low-pressure turbine 93. Steam discharged from the low-pressure turbine 93 is cooled by the condenser 95 to become condensed water. This condensed water is heated by the third heat exchanger 78, and returned to the economizers 51 and 52 after removal of remaining oxygen by the deaerator 96.

In this description, the third heat exchanger 78 is provided such that the heat exchange tube 78 a and the water/steam circulation path 94 are disposed close to each other to heat water supplied to the economizers 51 and 52 of the heat exchanging unit 14 by using heat recovered by the heat recovering unit 71. However, configuration other than this example may be adopted. As illustrated in FIG. 3, a third heat exchanger 79 may be provided to heat feed water supplied to the economizers 51 and 52 of the heat exchanging unit 14 by using heat recovered by the heat recovering unit 71, in a state that the water/steam circulation path 94 is connected with the third branch path 82 a and the fourth branch path 82 b.

The heat recovering unit 71 is hereinafter detailed. FIG. 4 is a view schematically illustrating a configuration of the heat recovering unit.

As illustrated in FIG. 4, the heat recovering unit 71 includes the high temperature unit 72 and the low temperature unit 73. The high temperature unit 72 is disposed in the second vertical path (vertical path) 44, while the low temperature unit 73 is disposed in the gas duct (horizontal path) 45. The NOx removal device 61 is provided in the second vertical path 44. The high temperature unit 72 is disposed below the NOx removal device 61 at a position away therefrom by a predetermined distance. The lower part of the second vertical path 44 is connected with a base end of the gas duct 45 while bended at right angles. A hopper 66 is provided in a connection portion (bended portion) between the second vertical path 44 and the gas duct 45. The hopper 66 is disposed below the NOx removal device 61 and the high temperature unit 72, and positioned on the side of the low temperature unit 73.

The heat recovering unit 71 is configured such that a path cross-sectional area of an outlet of the heat recovering unit 71 is made smaller than a path cross-sectional area of an inlet of the heat recovering unit 71. More specifically, an inlet 72A is formed in an upper part of the high temperature unit 72 positioned in the second vertical path 44, while an outlet 72B is formed in a lower part of the high temperature unit 72. On the other hand, an inlet 73A is formed at one end of the low temperature unit 73, while an outlet 73B is formed at the other end of the low temperature unit 73. Accordingly, an inlet of the heat recovering unit 71 constituted by the high temperature unit 72 and the low temperature unit 73 corresponds to the inlet 72A of the high temperature unit 72, while an outlet of the heat recovering unit 71 corresponds to the outlet 73B of the low temperature unit 73.

The path cross-sectional area in this context is an area of a cross section of the second vertical path 44 or the gas duct 45 in the flue gas duct 13 taken perpendicularly to the flow direction of flue gas. This cross-sectional area is an area through which flue gas is allowed to flow. More specifically, the path cross-sectional area is an area of a cross section taken perpendicularly to the flow direction of flue gas in the second vertical path 44 or the gas duct 45 after removal of closed portions by the heat exchange tube and fins.

In a specific configuration, an inner size of the outlet 73B of the heat recovering unit 71 in the flue gas duct 13 is made smaller than an inner size of the inlet 72A. Each of the second vertical path 44 and the gas duct 45 is constituted by a casing having a rectangular box-shaped cross section. The high temperature unit 72 contains the heat exchange tube 72 a, while the low temperature unit 73 contains the heat exchange tube 73 a. An inner size of the gas duct 45 is made smaller than an inner size of the second vertical path 44. Densities of the heat exchange tubes 72 a and 73 a of the high temperature unit 72 and the low temperature unit 73 are equalized. More specifically, the heat exchange tubes 72 a and 73 a are bended such that piping is curved a plurality of times. In this case, respective pipe portions are positioned adjacent to each other with predetermined distances left therebetween for arrangement at uniform intervals.

The inner size in this context is an inside area of the second vertical path 44 or the gas duct 45 in the flue gas duct 13. When the cross-sectional shape of the second vertical path 44 or the gas duct 45 is rectangular, the inner size is an area calculated as the product of the inner height and the inner width. When the cross-sectional shape of the second vertical path 44 or the gas duct 45 is circular, the inner size is an area calculated by the product of the diameter and the circular constant. In other words, the inner size is an area calculated without consideration of portions closed by the heat exchange tubes and fins disposed inside the second vertical path 44 or the gas duct 45.

Flue gas having passed through the NOx removal device 61 in the second vertical path 44 is high-temperature flue gas in the range from 300° C. to 400° C. While passing through the high temperature unit 72 of the heat recovering unit 71, the high-temperature flue gas heats the heat medium flowing in the heat exchange tube 72 a. As a result, the temperature and volume of the flue gas decrease. While passing through the low temperature unit 73 after the high temperature unit 72, the flue gas heats the heat medium flowing in the heat exchange tube 73 a. As a result, the temperature and volume of the flue gas decrease. The flue gas having passed through the heat recovering unit 71 (low temperature unit 73) in the gas duct 45 becomes low-temperature flue gas in the range from 80° C. to 120° C. On the other hand, the heat medium in the range from 65° C. to 100° C. introduced into the high temperature unit 72 of the heat recovering unit 71 is heated to 100° C. to 300° C., and discharged from the low temperature unit 73.

In this case, flue gas having a high temperature and a large volume passes through the high temperature unit 72 having a large path cross-sectional area in the second vertical path 44, while flue gas having a low temperature and smaller volume passes through the low temperature unit 73 having a small cross-sectional area in the gas duct 45. Accordingly, the flow velocity of the flue gas having the high temperature and the large volume does not increase while passing through the high temperature unit 72. On the other hand, the flow velocity of the flue gas having the low temperature and the small volume does not decrease while passing through the low temperature unit 73. In other words, the flow velocity of the flue gas is maintained substantially uniform with reduced fluctuations even when the temperature and volume of the flue gas gradually decrease during passage of the flue gas through the heat recovering unit 71.

As described above, the flue gas heat recovery system according to the first embodiment includes the furnace 11, the combustion device 12, the flue gas duct 13, and the heat exchanging unit 14. The flue gas heat recovery system further includes the heat recovering unit 71 disposed in the flue gas duct 13 on the downstream side of the heat exchanging unit 14 to recover heat from flue gas. The heat recovering unit 71 is configured such that the path cross-sectional area of the outlet 73B is made smaller than the path cross-sectional area of the inlet 72A. The flue gas heat recovery system further includes the first heat exchanger 74 which heats combustion air by using heat recovered by the heat recovering unit 71.

In this case, the fluctuations of the flow velocity of flue gas in the high temperature unit 72 and the low temperature unit 73 is reduced even when the temperature and volume of the flue gas decrease during passage through the heat recovering unit 71. As a result, the flow velocity does not become excessively high, wherefore erosion of the heat exchange tubes 72 a and 73 a constituting the heat recovering unit 71 are avoidable.

According to the flue gas heat recovery system of the first embodiment, the inner size of the outlet 73B of the heat recovering unit 71 in the flue gas duct 13 is made smaller than the inner size of the inlet 72A. According to this structure, the path cross-sectional area of the heat exchanger inlet 72A into which high-temperature flue gas flows is determined easily and in a simplified configuration, so that the flue gas flow velocity becomes a flow velocity appropriate for reducing erosion of the heat exchange tubes, and for efficiently recovering heat also from low-temperature flue gas after heat exchange on the upstream side.

According to the flue gas heat recovery system of the first embodiment, the heat recovering unit 71 includes the upstream side high temperature unit 72 and the downstream side low temperature unit 73. The path cross-sectional area of the low temperature unit 73 is made smaller than the path cross-sectional area of the high temperature unit 72. In the structure of the heat recovering unit 71 constituted by the high temperature unit 72 and the low temperature unit 73, the path cross-sectional area of the inlet 72A of the heat recovering unit 71 into which high-temperature flue gas flows is determined easily, so that the flue gas flow velocity becomes a flow velocity appropriate for reducing erosion of the heat exchange tubes, and for recovering heat efficiency also from low-temperature flue gas after heat exchange on the upstream side.

According to the flue gas heat recovery system of the first embodiment, the flue gas duct 13 includes the second vertical path 44 extending in the vertical direction, and the horizontal gas duct 45 connected with the lower part of the second vertical path 44 and extending in the horizontal direction. The high temperature unit 72 is disposed in the second vertical path 44, while the low temperature unit 73 is disposed in the gas duct 45. Accordingly, the path cross-sectional area of the heat exchanger inlet 72A into which high-temperature flue gas flows is easily determined so that the flue gas flow velocity becomes a flow velocity appropriate for reducing erosion of the heat exchange tubes, and for efficiently recovering heat also from low-temperature flue gas after heat exchange on the upstream side, without the necessity for changing the configuration of the existing flue gas duct 13.

According to the flue gas heat recovery system of the first embodiment, the NOx removal device 61 is provided in the second vertical path 44. The high temperature unit 72 is disposed below the NOx removal device 61. In this case, flue gas flows into the high temperature unit 72 after removal of harmful substances by the NOx removal device 61. Accordingly, adhesion of harmful substances to the heat recovering unit 71 is avoidable.

According to the flue gas heat recovery system of the first embodiment, the hopper 66 is provided between the second vertical path 44 and the gas duct 45 and below the high temperature unit 72. Accordingly, the hopper 66 disposed below the high temperature unit 72 is capable of collecting particles such as soot and dust contained in flue gas between the second vertical path 44 and the gas duct 45.

The flue gas heat recovery system of the first embodiment includes the second heat exchanger 77 which reheats flue gas prior to discharge from the stack 65 by using heat recovered by the heat recovering unit 71. The second heat exchanger 77 prevents generation of white smoke by reheating the flue gas using heat recovered by the heat recovering unit 71.

The flue gas heat recovery system according to the first embodiment includes the third heat exchanger 78 (79) which heats water supplied to the heat exchanging unit 14 by using heat recovered by the heat recovering unit 71. The third heat exchanger 78 (79) achieves effective utilization of recovered heat by using the heat recovered by the heat recovering unit 71 as heat for heating the water supplied to the heat exchanging unit 14.

Second Embodiment

FIG. 5 is a view schematically illustrating a heat recovering unit and heat exchangers included in a flue gas heat recovery system according to a second embodiment. Components in this embodiment having functions similar to the corresponding functions of the components in the foregoing embodiment have been given similar reference numbers, and detailed explanations of these components are not repeated herein.

As illustrated in FIG. 5, the flue gas heat recovery system according to the second embodiment includes a distribution amount control device which adjusts distribution amounts of a heat medium supplied from the heat recovering unit 71 to the first heat exchanger 74, the second heat exchanger 77, and the third heat exchanger 78.

The heat recovering unit 71 includes the high temperature unit 72 and the low temperature unit 73. The high temperature unit 72 and the low temperature unit 73 contain the heat exchange tubes 72 a and 73 a, respectively. One end of the heat exchange tube 72 a is connected with one end of the heat exchange tube 73 a. The other end of the heat exchange tube 72 a is connected with the first circulation path 75 a, while the other end of the heat exchange tube 73 a is connected with the second circulation path 75 b. The first circulation path 75 a and the second circulation path 75 b are connected via a bypass path 101. A flow rate control valve 102 is provided in the bypass path 101, while a flow rate control valve 103 is provided in the second circulation path 75 b on the low temperature unit 73 side with respect to a connection portion of the bypass path 101.

The first heat exchanger 74 contains the heat exchange tube 74 a. The first circulation path 75 a is connected with an end of the heat exchange tube 72 a. A flow rate control valve 104 is provided in the first circulation path 75 a on the first heat exchanger 74 side. The second heat exchanger 77 contains the heat exchange tube 77 a. The first branch path 81 a is connected with an end of the heat exchange tube 77 a. A flow rate control valve 105 is provided in the first branch path 81 a on the second heat exchanger 77 side. The third heat exchanger 78 contains the heat exchange tube 78 a. The third branch path 82 a is connected with an end of the heat exchange tube 78 a. A flow rate control valve 106 is provided in the third branch path 82 a on the third heat exchanger 78 side.

A temperature sensor 107 is provided to measure a temperature of the flue gas on the outlet 73B side of the low temperature unit 73 of the heat recovering unit 71. A temperature sensor 108 is provided to measure a temperature of the combustion air on the outlet side of the first heat exchanger 74. A temperature sensor 109 is provided to measure a temperature of the flue gas on the outlet side of the second heat exchanger 77. A temperature sensor 110 is provided to measure a temperature of the heat medium on the outlet side of the second heat exchanger 77. A temperature sensor 111 is provided to measure a temperature of the feed water on the outlet side of the third heat exchanger 78.

A control device 100 controls opening positions of the flow rate control valves 102, 103, 104, 105, and 106 based on the measurement results of the temperature sensors 107, 108, 109, 110, and 111 to adjust distribution amounts of the heat medium supplied from the heat recovering unit 71 to the respective heat exchangers 74, 77, and 78. Generally, the opening positions of the flow rate control valves 102 and 103 are adjusted such that the temperature of the flue gas on the outlet 73B side of the heat recovering unit 71 becomes a predetermined temperature (85° C. to 120° C.) to increase or decrease the supply amount of the heat medium to the respective heat exchangers 74, 77, and 78. The opening position of the flow rate control valve 104 is adjusted such that the temperature of the combustion air on the outlet side of the first heat exchanger 74 becomes a predetermined temperature (200° C. to 330° C.) to increase or decrease the supply amount of the heat medium to the first heat exchanger 74. The opening position of the flow rate control valve 105 is adjusted such that the temperature of the flue gas on the outlet side of the second heat exchanger 77 becomes a predetermined temperature (80° C. to 100° C.), and that the temperature of the heat medium on the outlet side of the second heat exchanger 77 becomes a predetermined temperature (80° C. to 95° C.), to increase or decrease the supply amount of the heat medium to the second heat exchanger 77. The opening position of the flow rate control valve 106 is adjusted such that the temperature of the feed water on the outlet side of the third heat exchanger 78 becomes a predetermined temperature (60° C. to 100° C.) to increase or decrease the supply amount of the heat medium to the third heat exchanger 78.

As described above, the flue gas heat recovery system of the second embodiment includes the flow rate control valves 102, 103, 104, 105, and 106, and the control device 100 for controlling these valves as a distribution amount control device for adjusting the distribution amounts of the heat medium supplied from the heat recovering unit 71 to the first heat exchanger 74, the second heat exchanger 77, and the third heat exchanger 78 (79)

Accordingly, the distribution amount control device makes it possible, while keeping a necessary amount of heat recovered from flue gas for environmental measurements, to utilize the remaining amount of heat recovered for improvement of power generation efficiency, by control of the distribution amounts of the heat medium supplied from the heat recovering unit 71 to the respective heat exchangers 74, 77, and 78 (79) in accordance with the operation condition of the boiler 10.

Third Embodiment

FIG. 6 is a view schematically illustrating a heat recovering unit and heat exchangers of a flue gas heat recovery system according to a third embodiment. Components in this embodiment having functions similar to the corresponding functions of the components in the foregoing embodiments have been given similar reference numbers, and detailed explanations of these components are not repeated herein.

As illustrated in FIG. 6, the flue gas heat recovery system according to the third embodiment includes a distribution amount control device which adjusts distribution amounts of the heat medium supplied from the heat recovering unit 71 to the first heat exchanger 74, the second heat exchanger 77, and the third heat exchanger 78.

The first heat exchanger 74 includes a bypass path 121 which connects respective ends of the heat exchange tube 74 a, i.e., the inlet and outlet of the heat exchange tube 74 a. A flow rate control valve 122 is provided in the bypass path 121. The second heat exchanger 77 includes a bypass path 123 which connects respective ends of the heat exchange tube 77 a, i.e., the inlet and outlet of the heat exchange tube 77 a. A flow rate control valve 124 is provided in the bypass path 123. The third heat exchanger 78 includes a bypass path 125 which connects respective ends of the heat exchange tube 78 a, i.e., the inlet and outlet of the heat exchange tube 78 a. A flow rate control valve 126 is provided in the bypass path 125.

The control device 100 controls opening positions of the flow rate control valves 122, 124, and 126 based on the measurement results of the temperature sensors 107, 108, 109, 110, and 111 to adjust distribution amounts of the heat medium supplied from the heat recovering unit 71 to the respective heat exchangers 74, 77, and 78. In other words, the control device 100 controls the flow rates of the heat medium bypassing the respective heat exchangers 74, 77, and 78 by adjusting the opening positions of the flow rate control valves 122, 124, and 126. Other control by the control device 100 for adjusting the distribution amounts of the heat medium is similar to the corresponding control in the second embodiment.

As described above, the flue gas heat recovery system according to the third embodiment includes the flow rate control valves 102, 103, 104, 105, 106, 122, 124, and 126, and the control device 100 for controlling these valves as the distribution amount control device for adjusting distribution amounts of the heat medium supplied from the heat recovering unit 71 to the first heat exchanger 74, the second heat exchanger 77, and the third heat exchanger 78.

Accordingly, it is possible to utilize appropriately heat recovered from flue gas for appropriate operation of the boiler 10 with the distribution amount control device adjusting the distribution amounts of the heat medium supplied from the heat recovering unit 71 to the respective heat exchangers 74, 77, and 78 in accordance with the operation condition of the boiler 10.

Moreover, the flue gas heat recovery system according to the third embodiment controls the flow rates of the heat medium bypassing the heat exchange tube 74 a of the first heat exchanger 74, the heat exchange tube 77 a of the second heat exchanger 77, and the heat exchange tube 78 a of the third heat exchanger 78 by adjusting the opening positions of the flow rate control valves 122, 124, and 126. Accordingly, the flue gas heat recovery system according to the third embodiment achieves efficient distribution of the heat medium in accordance with the amounts of heat required by the respective heat exchangers 74, 77, and 78.

Fourth Embodiment

FIG. 7 is a view schematically illustrating a heat recovering unit and heat exchangers included in a flue gas heat recovery system according to a fourth embodiment. Components in this embodiment having functions similar to the corresponding functions of the components in the foregoing embodiment have been given similar reference numbers, and detailed explanations of these components are not repeated herein.

As illustrated in FIG. 7, the flue gas heat recovery system according to the fourth embodiment includes a distribution amount control device which adjusts distribution amounts from the heat recovering unit 71 to the first heat exchanger 74, the second heat exchanger 77, and the third heat exchanger 78.

The first heat exchanger 74 contains the heat exchange tubes 74 a and 74 b. The first circulation path 75 a is connected with an end of the heat exchange tube 74 a. A bypass path 131 branched from the first circulation path 75 a is connected between the heat exchange tubes 74 a and 74 b. The flow rate control valve 104 is provided in the first circulation path 75 a. A flow rate control valve 132 is provided in the bypass path 131. The second heat exchanger 77 contains the heat exchange tubes 77 a and 77 b. The first branch path 81 a is connected with an end of the heat exchange tube 77 a. A bypass path 133 branched from the first branch path 81 a is connected between the heat exchange tubes 77 a and 77 b. The flow rate control valve 105 is provided in the first branch path 81 a. A flow rate control valve 134 is provided in the bypass path 133. The third heat exchanger 78 contains the heat exchange tubes 78 a and 78 b. The third branch path 82 a is connected with an end of the heat exchange tube 78 a. A bypass path 135 branched from the third branch path 82 a is connected between the heat exchange tubes 78 a and 78 b. The flow rate control valve 106 is provided in the third branch path 82 a. A flow rate control valve 136 is provided in the bypass path 135.

The control device 100 controls opening positions of flow rate control valves 132, 134, and 136 based on the measurement results of the temperature sensors 107, 108, 109, 110, and 111 to adjust distribution amounts of the heat medium supplied from the heat recovering unit 71 to the respective heat exchangers 74, 77, and 78. In other words, the control device 100 controls the flow rates of the heat medium bypassing the respective heat exchangers 74, 77, and 78 by adjusting the opening positions of the flow rate control valves 132, 134, and 136. Other control by the control device 100 for adjusting the distribution amounts of the heat medium is similar to the corresponding control in the third embodiment.

As described above, the flue gas heat recovery system according to the fourth embodiment includes the flow rate control valves 102, 103, 104, 105, 106, 122, 124, 126, 132, 134, and 136 and the control device 100 for controlling these valves as the distribution amount control device for adjusting distribution amounts of the heat medium supplied from the heat recovering unit 71 to the first heat exchanger 74, the second heat exchanger 77, and the third heat exchanger 78.

Accordingly, it is possible to utilize appropriately heat recovered from flue gas for appropriate operation of the boiler 10 with the distribution amount control device adjusting the distribution amounts of the heat medium supplied from the heat recovering unit 71 to the respective heat exchangers 74, 77, and 78 in accordance with the operation condition of the boiler 10.

Moreover, the flue gas heat recovery system according to the fourth embodiment contains the heat exchangers 74, 77 and 78 which are composed of two heat exchange tubes 74 a and 74 b, two heat exchange tubes 77 a and 77 b and two heat exchange tubes 78 a and 78 b respectively. And the flue gas heat recovery system selectively uses the heat exchange tubes 74 a and 74 b of the first heat exchanger 74, the heat exchange tubes 77 a and 77 b of the second heat exchanger 77, and the heat exchange tubes 78 a and 78 b of the third heat exchanger 78 for control of the flow rates of the heat medium by controlling the opening positions of the flow rate control valves 132, 134, and 136 provided between the two heat exchange tubes 74 a and 74 b of the heat exchanger 74, between the two heat exchange tubes 77 a and 77 b of the heat exchanger 77, and between the two heat exchange tubes 78 a and 78 b of the heat exchanger 78, respectively. Accordingly, the flue gas heat recovery system according to the fourth embodiment achieves efficient distribution of the heat medium in accordance with the amounts of heat required by the respective heat exchangers 74, 77, and 78.

Fifth Embodiment

FIG. 8 is a view schematically illustrating a heat recovering unit included in a flue gas heat recovery system according to a fifth embodiment, while FIG. 9 is a view schematically illustrating a fin tube of the heat recovering unit. Components in this embodiment having functions similar to the corresponding functions of the components in the foregoing embodiment have been given similar reference numbers, and detailed explanations of these components are not repeated herein.

As illustrated in FIG. 8, the heat recovering unit 140 of the flue gas heat recovery system according to the fifth embodiment is configured such that a path cross-sectional area of an outlet 140B of the heat recovering unit 140 is made smaller than a path cross-sectional area of an inlet 140A of the heat recovering unit 140. More specifically, the heat recovering unit 140 is provided in the gas duct 45 of the flue gas duct 13. The density of a heat exchange tube 140 a or heat exchange fins 140 b at the outlet 140B is set higher than the density of the heat exchange tube 140 a or the heat exchange fins 140 b at the inlet 140A.

For example, the density of the heat exchange fins 140 b is set higher in the direction from the inlet 140A to the outlet 140B of the heat recovering unit 140 as illustrated in FIG. 9. The heat exchange tube 140 a is bended such that piping is curved at a plurality of points. In this case, respective piping portions are disposed adjacent to each other with predetermined distances left therebetween for arrangement at uniform intervals. On the other hand, the heat exchange fins 140 b are fixed to an outer circumference of the heat exchange tube 140 a. The distances between the respective heat exchange fins 140 b decrease in the direction from the inlet 140A to the outlet 140B, while the number of the heat exchange fins 140 b increases in the direction from the inlet 140A to the outlet 140B. The heat exchange fins 140 b close a part of the gas duct 45. Accordingly, the flue gas path cross-sectional area of the heat recovering unit 140 decreases in the direction from the inlet 140A to the outlet 140B.

High-temperature flue gas heats a heat medium flowing in the heat exchange tube 140 a while passing through the heat recovering unit 140. As a result, the temperature and volume of the flue gas decrease, wherefore the flue gas having passed through the heat recovering unit 140 becomes low-temperature flue gas. According to this structure, flue gas having a high temperature and a large volume passes through the large path cross-sectional area on the inlet 140A side, while flue gas having a low temperature and a small volume passes through the small path cross-sectional area on the outlet 140B side. Accordingly, the flow velocity does not increase when the flue gas having the large volume and the high temperature passes through the inlet 140A. On the other hand, the flow velocity does not decrease when the flue gas having the small volume and the low temperature passes through the outlet 140B. In other words, fluctuations of the flow velocity are reduced even when the temperature and volume of the flue gas decrease during passage of the flue gas through the heat recovering unit 140.

According to the flue gas heat recovery system of the fifth embodiment, therefore, the density of the heat exchange tube 140 a or the heat exchange fins 140 b at the outlet 140B of the heat recovering unit 140 is set higher than the density of the heat exchange tube 140 a or the heat exchange fins 140 b at the inlet 140A.

Accordingly, the path cross-sectional area of the heat exchanger inlet 140A into which high-temperature flue gas flows is easily determined so that the flue gas flow velocity becomes a flow velocity appropriate for reducing erosion of the heat exchange tubes, and for efficiently recovering heat also from low-temperature flue gas after heat exchange on the upstream side, in accordance with a change of the position, shape, and number of the heat exchange tube 140 a or the heat exchange fins 140 b, without the necessity of changing the configuration of the flue gas duct 13.

Sixth Embodiment

FIG. 10 is a view schematically illustrating a heat recovering unit and heat exchangers included in a flue gas heat recovery system according to a sixth embodiment. Components in this embodiment having functions similar to the corresponding functions of the components in the foregoing embodiment have been given similar reference numbers, and detailed explanations of these components are not repeated herein.

As illustrated in FIG. 10, the flue gas heat recovery system according to the sixth embodiment includes the heat recovering unit 71 equipped with the high temperature unit 72 and the low temperature unit 73 for recovering heat from flue gas, the first heat exchanger 74 for heating combustion air by using heat recovered by the heat recovering unit 71, the second heat exchanger 77 for reheating flue gas discharged from the stack 65 by using heat recovered by the heat recovering unit 71, and the third heat exchanger 78 for heating feed water supplied to the economizers 51 and 52 of the heat exchanging unit 14 by using heat recovered by the heat recovering unit 71.

As illustrated in FIG. 10, an end of the heat exchange tube 72 a of the heat recovering unit 71 is connected with one end of the heat exchange tube 74 a of the first heat exchanger 74 via the first circulation path 75 a. The other end of the heat exchange tube 74 a of the first heat exchanger 74 is connected with one end of the heat exchange tube 77 a of the second heat exchanger 77 via a first connection path 151. The other end of the heat exchange tube 77 a of the second heat exchanger 77 is connected with one end of the heat exchange tube 78 a of the third heat exchanger 78 via a second connection path 152. The other end of the heat exchange tube 78 a of the third heat exchanger 78 is connected with an end of the heat exchange tube 73 a of the heat recovering unit 71 via the second circulation path 75 b.

In this case, the high-temperature heat medium after passing through the heat recovering unit 71 is supplied to the first heat exchanger 74 to heat air flowing through the air duct 38 and generate high-temperature combustion air. The high-temperature heat medium discharged from the first heat exchanger 74 is supplied to the second heat exchanger 77 to reheat flue gas. The high-temperature medium discharged from the second heat exchanger 77 is supplied to the third heat exchanger 78 to heat feed water.

As described above, the flue gas heat recovery system according to the sixth embodiment includes the first heat exchanger 74 for heating combustion air by using heat recovered by the heat recovering unit 71, the second heat exchanger 77 for reheating flue gas discharged from the stack 65 by using heat recovered by the heat recovering unit 71, and the third heat exchanger 78 for heating feed water supplied to the heat exchanging unit 14 by using heat recovered by the heat recovering unit 71. The first, second, and third heat exchangers 74, 77, and 78 are disposed in series.

In this case, the heat medium is sequentially delivered in accordance with necessary temperatures of the combustion air, flue gas, and feed water to achieve appropriate supply and heating of the heat medium. Accordingly, the configuration of the supply piping of the heat medium is simplified, wherefore the system is small-sized.

According to the first through third embodiments described herein, the heat recovering unit 71 includes the high temperature unit 72 and the low temperature unit 73. However, the number of the units included in the heat recovering unit 71 is not limited to two, but may be three or larger. According to the first through third embodiments described herein, the high temperature unit 72 is disposed in the second vertical path 44, while the low temperature unit 73 is disposed in the horizontal gas duct 45. However, both the high temperature unit 72 and the low temperature unit 73 may be disposed in the second vertical path 44, or in the gas duct 45. In this case, the inner size of the flue gas duct may be gradually decreased, or the density of the heat exchange tube or the heat exchange fins may be gradually increased.

According to the respective embodiments, the three heat exchangers 74, 77, and 78 are provided for the heat recovering unit 71. However, all of these heat exchangers are not required to be equipped. For example, such a structure which includes only the first heat exchanger 74 may be adopted.

REFERENCE SIGNS LIST

-   -   10 Boiler     -   11 Furnace     -   12 Combustion device     -   13 Flue gas duct     -   14 Heat exchanging unit     -   21, 22, 23, 24, 25 Combustion burner     -   36 Wind box     -   37 Additional air nozzle     -   38 Air duct     -   39 Blower     -   41 First horizontal path     -   42 First vertical path     -   43 Second horizontal path     -   44 Second vertical path     -   45 Gas duct     -   46, 47, 48 Superheater     -   49, 50 Reheater     -   51, 52 Economizer     -   61 NOx removal device     -   62 Electrostatic precipitator     -   64 Desulfurization device     -   65 Stack     -   66 Hopper     -   71, 140 Heat recovering unit     -   72 High temperature unit     -   72A, 73A, 140A Inlet     -   72B, 73B, 140B Outlet     -   73 Low temperature unit     -   74 First heat exchanger     -   76 Pump     -   77 Second heat exchanger     -   78, 79 Third heat exchanger     -   100 Control device     -   102, 103, 104, 105, 106, 122, 124, 126, 132, 134, 136 Flow rate         control valve (distribution amount control device)     -   107, 108, 109, 110, 111 Temperature sensor 

1. A flue gas heat recovery system for a boiler including a hollow furnace located in a vertical direction, a combustion burner that is disposed in a lower part of a wall constituting the furnace and blows fuel gas constituted by a mixture of fuel and combustion air toward the furnace, a flue gas path connected to an upper part of the furnace, in which a flue gas discharged from the furnace flows, and a heat exchanging unit disposed in the flue gas path to exchange heat between the flue gas and water from a condenser, comprising: a heat recovering unit disposed in the flue gas path on the downstream side of the heat exchanging unit to recover heat of flue gas and to heat a heat medium by the flue gas; a first heat exchanger that heats the combustion air by using heat recovered by the heat recovering unit; and a first heat medium circulation path, in which the heat medium circulates between the heat recovering unit and the first heat exchanger, wherein a cross-sectional area of the flue gas pass at an outlet of the heat recovering unit is configured to be smaller than a cross-sectional area of the flue gas pass at an inlet of the heat recovering unit.
 2. The flue gas heat recovery system according to claim 1, wherein the heat recovering unit is configured such that an inner size of the outlet in the flue gas path is made smaller than an inner size of the inlet.
 3. The flue gas heat recovery system according to claim 1, wherein the heat recovering unit is configured such that density of a heat exchange tube or heat exchange fins at the outlet is made higher than density of the heat exchange tube or the heat exchange fins at the inlet.
 4. The flue gas heat recovery system according to claim 1, wherein the heat recovering unit includes a high temperature unit and a low temperature unit disposed at a downstream side of the high temperature unit, and is configured such that a path cross-sectional area of the low temperature unit is made smaller than a path cross-sectional area of the high temperature unit.
 5. The flue gas heat recovery system according to claim 4, wherein the flue gas path includes a vertical path extending in the vertical direction and a horizontal path connected with a lower part of the vertical path and extending in a horizontal direction, and the high temperature unit is disposed in the vertical path, and the low temperature unit is disposed in the horizontal path.
 6. The flue gas heat recovery system according to claim 5, wherein a NOx removal device is provided in the vertical path, and the high temperature unit is disposed below the NOx removal device.
 7. The flue gas heat recovery system according to claim 5, wherein a hopper is provided between the vertical path and the horizontal path and below the high temperature unit.
 8. The flue gas heat recovery system according to claim 1, further comprising a second heat exchanger that is disposed between the heat recovering unit and a stack on the flue gas path to reheat the flue gas prior to discharging from the stack by using heat recovered by the heat recovering unit.
 9. The flue gas heat recovery system according to claim 8, further comprising: a third heat exchanger that is disposed between the heat exchanging unit and the condenser to heat the water supplied from the condenser and supplied to the heat exchanging unit by using heat recovered by the heat recovering unit.
 10. The flue gas heat recovery system according to claim 9, further comprising a distribution amount control device including a flow rate control valve and a control device that controls the flow rate control valve for adjusting distribution amounts of a heat medium supplied from the heat recovering unit to the first heat exchanger, the second heat exchanger, and the third heat exchanger.
 11. The flue gas heat recovery system according to claim 2, wherein the heat recovering unit is configured such that density of a heat exchange tube or heat exchange fins at the outlet is made higher than density of the heat exchange tube or the heat exchange fins at the inlet.
 12. The flue gas heat recovery system according to claim 2, wherein the heat recovering unit includes a high temperature unit and a low temperature unit disposed at a downstream side of the high temperature unit, and is configured such that a path cross-sectional area of the low temperature unit is made smaller than a path cross-sectional area of the high temperature unit.
 13. The flue gas heat recovery system according to claim 3, wherein the heat recovering unit includes a high temperature unit and a low temperature unit disposed at a downstream side of the high temperature unit, and is configured such that a path cross-sectional area of the low temperature unit is made smaller than a path cross-sectional area of the high temperature unit.
 14. The flue gas heat recovery system according to claim 6, wherein a hopper is provided between the vertical path and the horizontal path and below the high temperature unit.
 15. The flue gas heat recovery system according to claim 2, further comprising a second heat exchanger that is disposed between the heat recovering unit and a stack on the flue gas path to reheat the flue gas prior to discharging from the stack by using heat recovered by the heat recovering unit.
 16. The flue gas heat recovery system according to claim 3, further comprising a second heat exchanger that is disposed between the heat recovering unit and a stack on the flue gas path to reheat the flue gas prior to discharging from the stack by using heat recovered by the heat recovering unit.
 17. The flue gas heat recovery system according to claim 4, further comprising a second heat exchanger that is disposed between the heat recovering unit and a stack on the flue gas path to reheat the flue gas prior to discharging from the stack by using heat recovered by the heat recovering unit.
 18. The flue gas heat recovery system according to claim 5, further comprising a second heat exchanger that is disposed between the heat recovering unit and a stack on the flue gas path to reheat the flue gas prior to discharging from the stack by using heat recovered by the heat recovering unit.
 19. The flue gas heat recovery system according to claim 6, further comprising a second heat exchanger that is disposed between the heat recovering unit and a stack on the flue gas path to reheat the flue gas prior to discharging from the stack by using heat recovered by the heat recovering unit.
 20. The flue gas heat recovery system according to claim 7, further comprising a second heat exchanger that is disposed between the heat recovering unit and a stack on the flue gas path to reheat the flue gas prior to discharging from the stack by using heat recovered by the heat recovering unit. 